Within packaging technology, use has long been made of packages for packing and transporting products such as milk, juice and other beverages. A large group of these packages is produced from a laminated packaging material comprising a core layer of, for example, paper or paper-board and an outer, liquid-tight coating of thermoplastic material on at least that side of the core layer which forms the inside of the package. Sometimes the material also includes a gas barrier, for example in the form of an aluminum layer.
Such packaging containers are often produced in that a web of packaging material is formed into a tube before sealing the longitudinal overlapping edges of the web. The longitudinally sealed tube is continuously filled with a product and then transversally sealed and formed into cushions. The sealing is made along narrow, transverse, mutually spaced apart, sealing zones. The transverse sealing of the tube takes place in a per se known manner substantially at right angles to the longitudinal direction of the tube and constantly in the same plane. The sealed-off portions of the tube thus containing contents are thereafter separated from the tube by means of incisions in these sealing zones.
The above mentioned forming of the web into a tube is effected by means of a number of tube forming means, usually rings, through which the web passes. When passing the last tube forming means, the tube is longitudinally sealed after which the tube is ready for filling and subsequent transverse sealing. During the transverse sealing, the tube is squeezed between two opposing jaws heating the thermoplastic material in the sealing zone. The location of the longitudinal seal in relation to the transverse seal on the final package will affect its appearance and function and should be essentially the same on every package of a certain kind. The twist of the tube in relation to the jaws in the moment of transverse sealing will determine this relative location of the longitudinal seal on the final package. For example, it is often desirable to have a certain décor on the packages, such as pictures and information describing the contents of the product. Further, since the area where the longitudinal seal intersects the transverse seal will contain three layers of packaging material instead of the common number of two, this area will be a critical one when it comes to sealing. The jaws may therefore be constructed in such a way as to provide for an increased heating in the intersection area. Therefore, the position of the longitudinal seal is crucial.
Due to different factors, such as web tension, unwanted twists of the tube may occur during manufacture. In a known machine for producing packages like the ones described above, such a twist can be adjusted prior to tube forming and longitudinal sealing by lightly pushing one of the longitudinal edges of the web. However, this method for adjusting the twist of the tube is most suitable in connection with use of relatively thick and rigid packaging materials. In a case where a thinner and less rigid material is used, there may be a risk that the web is being deformed by the pushing. Further, the adjustment of the twist is effected well in advance of the longitudinal sealing. Therefore, there may be a risk that a new unwanted twist of the tube is introduced after the pushing adjustment and before the longitudinal sealing.